Electromagnetic-wave-receiving system



July 14, 1925:

S. CABOT ELECTROMAGNETIC WAVE RECEIVING SYSTEM "Filed May w arr Patented July 14, 1925 P'IATENT OFFICE.

SEWALL CABOT, 0F BROOKLINE, MASSACHUSETTS.

'ELEcTRoMAGNETIc wAvE-REcEIvING SYST EM.

Application filed May 25,

To all whom it may concern:

Be it known that I, SEWALL (hear, a citi- Zen of the United States, and a resident of Brookline, in the county of Norfolk and State of Massachusetts, have invented a new and useful Improvement in Electromagnetic-Wave-Receiving Systems, of which the following is a specification.

My invention relates to electromagneticwave receiving-systems and more especially to an-apparatus for controlling the oscillatory phenomena in such systems of the radio-frequency amplifier type.

The general object of my invention is to provide an electromagnetic-wave receivingsystem of the-radio-frequency-amplifier type in which the plate and grid circuits of a three-electrode vacuum tube are not exteriorly coupled or interlinked but are, in fact, shielded from each other, being coupled or interlinked only by the capacity between the plate and the grid, and are not provided with means supplementing the electrostatic coupling of the vacuum tube to facilitate the transfer of energy from the plate to the grid circuit or to increase the effect upon the grid of the high frequency pulsations in the plate circuit. In the electromagnetic-wave receivingsystems of the regenerative-detector type now in general use, various methods have been employed for controlling the creation of local oscillations, for example, the method of so varying the coupling between the plate and grid circuits that the system is just on the verge of creating such local oscillations, this being the condition in which the system is most sensitive to feeble electrical oscillations.

Such method of so controlling the creation of local oscillations is open to the objection that any variation in the coupling between the plate and grid circuits varies the natural period of the oscillatory receiving-circuit, thereby necessitating the retuning of such circuit for feeble electrical oscil lations or weak signals every time the coupling between the plate and grid circuits is varied.

When a. tuned radio-frequency amplifier consisting of two parallel-branch circuits each having an inductance in one branch and a condenser in the other and connected, respectively, between the grid and the negative end of the filament, and between the 1922. Serial in. 563,611.-

plate and filament, is so adjusted that said two circuits are approximately in tune, strong local oscillations will be created if the product of inductance by capacity of the first, or grid, circuit slightly exceeds the product of inductance by capacity of the second, or plate, circuit. Such oscillations are detrimental because they are much too strong for advantageous reception of signals, especially weak signals, and various means have heretofore been employed to suppress them, such for example as a biasing potential put on the grid or a relatively large artificial resistance introduced into the circuits. l

Such means for suppressing the local oscillations are open to various objections including the unnecessary complication of the system, the difficulty of maintaining such system in permanent adjustment, and dimin ished sensitiveness'.

When a tuned radio-frequency amplifier is adjusted to create strong local oscillations in .the manner aforesaid, I have discovered that if a vernier adjustment of either of the tuning elements in the plate circuit be made so that a relatively large movement of its adjusting handle makes only a very small,

change in thetum'ng of said plate circuit, said local oscillations may be gradually reduced in amplitude until they finally cease, provided, however, that such adjustment is made so as to increase the value of either of the tuning elements in the plate circuit, and thereby increase the natural period of said circuit from that at which the strongest oscillations are created. When the oscillations have been suppressed by such increase in the natural period of the oscillatory platecircuit, the system is most sensitive to feeble electrical oscillations or weak signals.

It means are provided for simultaneously adjusting both of said circuits by one motion to approximate resonance to any frequency within their tuning range, the vernier tuning element may be very small, and the adjustment aforesaid may be made without disturbing the tuning of the grid circuit, and in this manner the tuning of the system andthe control of the oscillatory phenomena 105 are rendered substantially independent, thereby great-1y facilitating the adjustment of a system of this type for receiving feebleto provide a method of and means for controlling the creation of local oscillations without varyin the'natura-l period of the oscillatory recelving circuit, thereby rendering unnecessary the retuning of such circuit for feeble electrical oscillations of a given frequency, and without having recourse to the use of biasing potentials or artificial resistances.

Another object of my invention is to provide means for adjusting an electromagneticwave receiving-system to selectively receive feeble electrical oscillations or weak signals by two mutually independent motions only, one motion serving simultaneously to attune the oscillatory circuits approximately to the frequency of the oscillations to be received and the other motion serving to so adjust the, system that it is just on the verge of creating local oscillations, this, as aforesaid,

being the condition in which the system is most sensitive to feeble electrical oscillations or weak signals.

A further object of my invention is to provide a unitary means whereby one of the tuning elements, for example, the condenser, of each of said oscillatory circuits may be simultaneously varied, whereby the inductance of one of said oscillatory circuits may also be varied at the same time.

A further object of my invention is to provide a micrometer tuning element, for example, a vernier condenser, associated with one of the tuning elements of one of the oscillatory receiving circuits or two such mlcrometer tuning elements, associated, re-

- spectively, with one of the tuning elements of each of the oscillatory receiving circuits, whereby the natural period of such oscillatory circuit or circuits may be very slightly varied without altering the main tuning element or elements thereof.

Inasmuch as the theory underlying the regenerative amplification obtained by my invention is complex and obscure and any explanation thereof would at best be merely tentative. I deem it unnecessary herein to enter into a detailed statement of the probable nature of such action, but believe it to be substantially as follows: The control of the oscillatory phenomena is transferred from the detector circuit to the circuit of the radio-frequency amplifier, in which circuit the control of the oscillatory phenomena, i. e., the property of the system to create in ternal oscillations, is attained, not by varying the external coupling of the plate and gridcircuits of the audion, but by suppress ing this coupling and by adjusting the balance between the positive resistance of the circuits through which the local oscillations take place and which may be varied within wide limits by a slight adjustment of the capacity or inductance of said circuits when and, preferably,"

the latter are nearly resonant to the frecoil antenna, a three-electrode vacuum-tube interposed between said parallel 'branch c1rcuits, an oscillation detector, which may be a vacuum tube, crystal, etc, associated-with the second parallel-branch circuit, a signalindicating device associated with said oscillation detector, and a vernier tuning element associated with one of the tuning elements of one of said parallel-branch circuits, preferably the second of said parallel-branch cir cuits, or two such vernier tuning elements associated, respectively, with one of the tuning elements of each of said parallel-branch circuits, whereby the natural period of one or both of said circuits may be very slightly varied without altering the adjustment of the main tuning elements thereof, such vernier tuning element being a condenser, in

which case it is connected in parallel with the condenser in one of said parallelbranch circuits, preferably the second parallelbranch circuit, or if two are employed, they are connected, respectively, in parallel wlth the condensers of each .of said parallelbranch circuits.

My invention comprises also a unitary means, such as a rotatable rod, whereby one of the tuning elements, for example, a condenser, of each of said parallel-branch circuits' may be simultaneously varied, and

whereby at the same time the other tuning element, such as the inductance, of one of the parallel-branch circuits, preferably the second, may be varied.

My invention further comprisesa tuned radio frequency amplifier having its vacuum tube and the oscillatory circuits which are connected to its in-put and out-put circuits, respectively, so constructed and arranged that between the maximum and minimum.

adjustment of a vernier tuning element associated with one of the tuning elements of one of said oscillatory circuits local oscillations will be created and suppressed wherebv the system will be put in the most sensi-.

tive condition for receiving feeble electrical oscillations without altering the natural period of the receiving oscillatory circuit connected to the grid of said vacuum tube.

My invention further comprises an appa-I system of the radio-frequency amplifier type by varying the natural period of one of the two oscillatory circuits connected respectively to the grid and plate of a three-elec-- trode vacuum tube, for example, increasing such natural period in the plate circuit until 'the'local oscillations are suppressed without altering the natural periodof the receiving oscillatory circuit, thereby placing the system in the condition in which it is most Hal , sensitive to feeble electrical oscillations.

in using said apparatus, preferably one motion serves simultaneously to attune the two oscillatory circuits approximately -,to the frequency of the waves the energy of which is to be received,'whereupon the natural period of theplate-oscillatory circuit may be slightly reduced by an independent mosuppressed, thereby attaining the sensitive condition aforesaid; or, the system may be so adjusted that local oscillations will be created at the time the twooscillatory circuits are brought approximately into time with the signal wave by, a single motion-on the part of the operator, in which case the local plate P Preferably the'indu-ctance L is a coil aerial consisting of a coil having relatively large area with its turns suitably spaced apart to reduce its resistance, and the condenser C is a variable air condenser, preferably of the rotary-plate type, serving to vary thenatural period of the oscillatory circuit L, 0,, and thereby attune said circuit to the, frequency of the oscillations to be i received.

The out-put circuit of the amplifier audion A is connected to the parallelbranch having the inductance L, in one branch and the, condenser C in the other, said inductance being shown in the present instance as a variometer and said condenser as a .variable-air-condenser which, preferably, is of the rotary-plate tvpe.

As indicated in the drawings, the in-put circuit of the detector audion A, havingthe filament F grid G and plate P is con- "nected; -to the second parallel-branch circuit L, Q, the grid circuit of said audion being provided with the usual condenser C and the usual leak resistance R being connected between the grid and filament; but it will be understood ofcourse that an oscillation detector of any other type, such as a crystal detector, may be suitably connected with said second parallel-branch circuit.

it will be obvious that instead of connecting a detector, such as the tube A. to

oscillations will be suppressed as before byn the second parallel-branch circuit 1L G a slightly increasing the natural period of the plate oscillatory circuit without altering that of the other and the aforesaid sensitivecondition attained.

The drawing which accompanies and forms a part of this specification represents conventionally and for illustrative purposes only an arrangement of circuits and apparatus embodying myinvention, and whereby my method may be realized in practice;

but it will be understood that various modifications may be made both in the apparatus and circuit arran ements therein shown without departing om my invention as delined by the appended claims andalso that roader than more a paratus by a multip ICltY-Of .vention and which is to be'understood .as illustrative rather than. restrictive, a para-l lei-branch circuit having an inductance ll/1 in one branchand a condenser 5C, in-the other is connected to tlui'imput circuit of the three-electrode yacuumftube or audion A; having the filament.' F G quency amplifier or other multi-stage radio- 105, I

frequency amplifier, each parallel-branch circuit being provided with one of the vernier tuning elements hereinafter described;

an that,"in such case, a detector will associated with the parallel-branch circilit' which is the most remote from the amplifier tube; and that any silitable audio-frequency amplifier of the sin le or multi-stage type maybe associated with-such detector.

'In the present instance, a slgnal-indl- 1 catingdeviceT of any suitable type, such as a high-resistance telephone, is associated. withxthe oscillation detector A, by connectv ingj itt o the out-put circuit of said detector; said. signal-indicating device being shunted by the usual condenser C The battery B is connected through the variable resistances or potentiometers R, R to the filaments F F and the out-put circuits of the tubes include the usual bat 1125 teries B B respectively.

it is desirable to' provide a unitary means for..;sin'1ultaneously varying the naturaleriods ofthe parallel-branchcircuits L C C,,;and while it willbe obvious to those skilled in the art that various means maybe employed for this purpose, I prefer, as indicated in dotted lines, to mount the rotary glates of the condensers, C 0,, on theshaft rotatable by the knob The movable element'of the variometer L, may beso arrangedfor relative movement with respect to' -,the stationary element thereof that the in-' 'ductance of said variometer may be varied.

simultaneously with 'the variation in thecapacity of the condensers C C As indicated in dotted lines, the said movable element of the variometer L is mounted on the shaft S, although it will readily occur to those skilled in the art that a multiplicity of means may be emplo ed for varying the inductance of the circult L, (3,, simnltaneousl'y with the variation of the capacity of the condensers G C Preferably the maximum inductance of the variometer L or such other inductance element as may be employed in lieu thereof, is much lar er than the inductance of the coil L and t e movable element thereof is so arranged'on the shaft S that the inductance of said variometer is a miximum at such position of the shaft as causes the condenser C to have its'maximum capacity.

The rotary plates of the condenser C, are so shaped that for any given sition of the shaft S the product of the in uctance of the circuit L,, Q, by the. ca acity thereof is equal to the product of t e inductance of the circuit L C", by the capacity thereof. The shape of the-irotary plates of the condenser 2 may bedetermined approximately from thecalibration curve of the variometer L, and the final-adjustment of such shape may be determined by the fact that the system will oscillate only when the two circuits L C, and L '0, are approximately in tune,

the presence of oscillations being detectable 4 by a milliammeter'M in the out-put circuit of the tube A.,. The plates of the condenser C preferably are made: slightly too large at first and then are pared down to exact shape so that a slight movement of the vernier con denser C, from its half-value position in the direction of a decreasedcapacity causes oscillations to start; r

Associated with one of the 1 tuning elements of one or both of the parallel-branch condensers 0' C1, connected, n parallel with the condensers circuits is a vernier tuning element, and in y the present instance I haveiishown Vernier pectively, v i 15- C? of sa1d clrcuits, although as hereinafter pointed out the'conde'nser C is not absolutely essen- 1 maximum capacity of the condenser O tial.. Thevernier condenser C may have a maximum capacity of about one-tenth of the Without further description or illustration it-will be readily apparent to those skilled in the artythat in lieu; of said vernier condenser C, cennected' in parallel with the condenser C,, a vernier or micrometer inductance may be connected in sense with the inductance element 11 When the lates ofthe condenser C, have been given t e'correct shape by the method above pointed out or by any other suitable method, local oscillations may be created in thereceiving system for any position of the shaft S, or in other words, for any frequency to which it is attuned, by slightly decreasing the capacity of the Vernier condenser C,, and such oscillations may be checked.

by slightly increasing the capacity of said Vernier condenser.

A receiving system constructed in accordance with my invention is so highly selective When used in the manner above set forth that a very small movement of the shaft S will quickly detune it, It is therefore advisable to employ a Vernier tuning element 8 associated with one of the tuning elements of the parallel-branch circuits L 0,. the present instance, such Vernier tuning element is represented as a vernier condenser C connected in parallel with the condenser (3,, although it will readily be understood without-further explanation or illustration that in lieu of such ver'nier condenser a vernier or micrometer inductance may be connected in series with the inductance L,.

It is not, however, absolutely essential to employ a Vernier tuning element in connection with the circuit L C, to make delicate adjustments in the period of said circuit, for, as will be readily a parent to those skilled in the art, the movab e plates of the condenser C, may be actuated by a reduction-gear driving-handle for rotating the shaft S through a very small arc to make the final adjustment. I v o Generally speaking, the changes in the natural periods of the'parallel-branch circuits L 0 L (3,, effected by the Vernier tuning elements is of the order of one-half of one percent. a It is highly desirable to shield the plate and grid circuits of the vacuum tube, A

from each other, so that there shall be no exterior coupling between said circuits, and

if twosuch' vacuum tubes are employed their plate and grid circuits should be similarly shielded. In the present instance the means employed for this purpose are represented the and plate circuits of the tube or of either of the tubes, if two are employed, efiective control of the oscillatory phenomena as above set forth becomes impaired,

.the oscillations having a tendency to start and stop suddenly so that the system either will not oscillate at all or else will oscillate sostrongly that its sensitiveness to feeble electrical oscillations is reduced.

I have determined experimentally that when the circuits of the system are attuned in the manner above set forth and the relations above specified between the several elements are obtained, a. very considerable amplification is effected and without the use of a receiving antenna relatively feeble electromagnetic signal-waves transmitted over long distances may readily be received.

Having thus described an illustrative embodiment of my invention and electrical means whereby my method may be practiced, without however, limiting the same thereto, what I claim and desire to secure by Letters Patent is 1. An electromagnetic-wave receiving-system comp-risin in combination a parallelbranch circuit aving an inductance in one branch and a condenser in the other, a threeelectrode vacuum tube having its in-put circuit connected to said parallel-branch circuit, a second parallel-branch circuit having an inductance in one branch and a condenser in the other, means connecting the out-put circuit of said vacuum tube to said second parallel-branch circuit, means for simultaneously adjusting both of said circuits by one motionto approximate resonance to any frequency within their tuning range, and a vernier tuning element associated with one of the tuning elements of one of said parallel-branch circuits for controlling the generation and suppression of local oscillations in the system without altering the natural period of the other of said parallel-branch circuits whereby the tuning of the system and the control of the oscillatory phenomena are substantially independent. c

'2., An electromagnetic-wave receiving-system comprisin in combination a parallelbranch circuit liaving an inductance in one branch and a condenser'in the other, a three- Vernier tuning element asociated with one of the tuning elements of said second parallel-branch circuit for controlling the generation and suppression of local oscillations in the system without altering the natural period of the first mentioned parallelbranch circuit whereby the tuning of the system and the control of the oscillatory phenomena are substantially independent.

3. An electromagnetic-wave receiving-system comprising in combination a parallelbranch circuit having an inductance in one branch and a condenser in the other, athreeelectrode vacuum tube having its in-put circuit connected to said parallel-branch circuit, a second parallel-branch circuit having an inductance in one branch and a condenser in the other, means connecting the output circuit of said vacuum tube to said second parallel-branch circuit, movable means for mechanically connecting a tuning element of one of said parallel-branch circuits with a tuning element of the other of said parallel-branch circuits, said tuning elements being so constructed and arran ed that for any given position of said movajile means both parallel-branch circuits are attuned to substantially the same frequency, and means for slightly varying the natural period of one of said parallel-branch circuits without alterin that of the other of said parallel-branch circuits whereby the oscillatory phenomena of said parallel-branch circuits may be controlled independently of the position of said movable means. a

In testimony whereof, I have hereunto subscribed my name this 23d day of May, 1922. a

SEWALL CABOT. 

